(1) Field of the Invention
This invention relates to rectified and smoothed DC supplying circuitry for a B-class audio power amplifier or the like.
(2) Description of the Prior Art
A typical example of the conventional circuitry for supplying rectified and smoothed DC power for instance to a B-class audio power amplifier is shown in FIG. 1, in which a transformer 1 is given an input to its primary winding 1a from a commercial AC power source 2. The AC output of its secondary winding 1b is rectified by a rectifier circuit 3 and the rectified output is supplied to a smoothing circuit S having two capacitors 4, 5, more particularly across a positive-end terminal 4a of a positive-side capacitor 4 and a negative-end terminal 5b of a negative-side capacitor 5. Both these terminals 4a, 5b are made here to serve as the positive and negative output DC terminals, respectively. To the center junction of these serially connected capacitors 4, 5 there is connected an end of a center-tap lead circuit C of the secondary winding 1b.
When the thusly constructed power supply circuitry is supplying to the loads the respective currents I+ and I- as illustrated, there accordingly flow charging currents I.sub.1 and I.sub.2 of the capactitors 4 and 5, respectively, of intensity generally proportional to the load currents I+ and I-. Through the secondary winding 1b of the transformer 1 in such operation, there flow both the load currents and the charging currents, and in view that the charging of each capacitor 4 or 5 is attributed to a respective half side of the secondary winding 1b as delimited by the center tap 6, the respective secondary winding currents are referred to as i.sub.1 and i.sub.2 as shown in FIG. 1. Accordingly, the intensity of the current flowing through the center-tap lead circuit C is the balance of the respective secondary winding currents i.sub.1, i.sub.2, thus i.sub.1 -i.sub.2. Thus, in the respective half sides of the secondary winding 1b, as delimited by the center tap 6, there flow currents having intensity different with each other. Even if the load currents I+ and I- are the same in intensity the one as the other, the respective secondary winding currents i.sub.1, i.sub.2 actually are seldom of equal intensity, on account of the irregularity as is unavoidable between both the capacitors 4, 5 and also the irregularity between the respective half sections of the secondary winding 1b delimited by the center tap 6, or still further the like.
Though not clear from FIG. 1, it is supposed that both the positive-side and negative-side loads have considerable inductance. Under such supposition, and further supposing the case where the center-tap lead circuit C had been omitted or blockedly interrupted, then dangerous voltages far higher than the output AC peak voltages of the secondary winding half sections might occur across the respective capacitors 4, 5 if and when the respective loads should show extravagantly rapid fluctuation by some reason or other, and would destroy the capacitors 4, 5 which are normally designed to endure rather moderate maximum voltages, thus of only moderate degree of safety coefficient. However, the conventional circuitry of FIG. 1 is actually provided, as has been described hereinbefore, with the center-tap lead circuit C which functions to safely draw out such extraordinarily increasing voltages across the respective capacitors, and this function is therefore the merit of the conventional circuitry of this type.
Referring here to FIG. 2, consider the case where the power supply circuitry 7 has as its load a power amplifier 8 for amplifying an input audio signal from a signal source 9 and reproducing the sound by means of a loudspeaker 10. If the frequency of the input signal is lower than the frequency of the power source 2 (FIG. 1), then the charging currents I.sub.1, I.sub.2 of the capacitors 4, 5, respectively, fluctuate in the frequency identical with that of the input signal but in 180.degree. out-of-phase relationship in the signal frequency with respect to each other. Consequently, the capacitor-charging currents, center-tap lead circuit current, secondary winding current and primary winding current have all of them the respective components of the input signal frequency. On the other hand, even if the input signal frequency is sufficiently higher than the power source frequency, the said respective currents have as yet the components of the input signal frequency, on account of the irregularities, as mentioned hereinbefore, of the various constituent parts.
FIG. 3(a) shows an example of 10 Hz input signal wave form, thus of the frequency far lower than that of the power source to be supposed as 60 Hz. When such signal is amplified by the conventional power amplifier to reproduce the sound by the loudspeaker (speaker output voltage of 3,162 V across the impedance of 8.OMEGA.), per the system of FIG. 2, then the secondary winding current shows the wave form upon passing through a low-pass filter, thus deprived of the higher frequency components beyond 500 Hz, as FIG. 3(b). The frequency spectrum thereof is shown in dot line charting in FIG. 4.
The same input signal wave form is again given as FIG. 5(a), and shown in contrast herewith as FIG. 5(b) directly thereunder is the wave form of the center-tap lead circuit current, also as deprived of the higher frequency components beyond 500 Hz, while the frequency spectrum thereof is shown in dot line charting in FIG. 6.
From these actually measured results, it has thus experimentally been proved that in the power source transformer secondary winding current and the center-tap lead circuit current there are included not only the amplitude information of the input signal but also the signal frequency components, of which particularly conspicuous are the components of the added sum and the subtracted balance frequencies as derived from the signal fundamental frequency f.sub.1 on the one hand and the harmonics of the power source frequency f.sub.2 in the order of even numbers on the other hand.
Situation is also just similar as to the power source transformer primary winding current, of which the frequency spectrum is represented by dot line charting in FIG. 7, showing as well that there are many mixed-up modulation components as derived from the signal fundamental frequency f.sub.1 and the power source frequency f.sub.2.
As in seen from the above, the power source transformer primary winding current and secondary winding current, capacitor currents and center-tap lead circuit currents in such conventional power supply circuitry include not only the input signal amplitude information but also the signal (frequency) information, and in this consequence the use of such circuitry has resulted in distortion on account that such signal information may sneak either directly into the circuits as are immediately connected to the power source circuit or else in the case of other circuits having no such immediate connection then indirectly through the intermediary of the function of electromagnetic induction.